Reverse bipolar transistor conduction in high-current PWM inverters

Most PWM (pulse width-modulated) converters using bipolar transistors experience the problem of reverse transistor conduction. This effect is analyzed for various types of base drives. It is shown to be the most serious for low-impedance direct drives. A model is developed for this case and verified experimentally. Problems associated with turn-on and turn-off in the reverse conduction mode are investigated. Various base drives, including both direct drives and Darlington configurations, are analyzed. Reverse transistor conduction is found to have the most serious implications for transistors driven directly by a low impedance source. Although discrete devices are discussed, the results apply to integrated Darlington modules as well.